Heretofore, there have been known retinal cameras, wherein an illumination optical system having an illumination light source for illuminating the retina of an eye to be tested is provided with a ring-shaped slit aperture. This ring-shaped slit aperture is disposed to a position substantially conjugate to a pupil of the eye to be tested. This ring-shaped slip aperture is adapted to form a ring-shaped slit illumination light in the vicinity of the pupil. An illumination light emitted from the illumination light source becomes a ring-shaped slit illumination light in the vicinity of the pupil of the eye to be tested, and the retina of the eye to be tested is illuminated by the ring-shaped slit illumination light. The illumination light reflected by the retina of the eye to be tested is guided to a photographing optical system for photographing the retina. The photographing optical system is prevented from entering therein a harmful light reflected by the cornea, anterior face of the crystal, etc. of the eye to be tested, since the retina is illuminated by the ring-shaped slit illumination light.
However, the conventional retinal cameras have presented such inconveniences as that the brightness is decreased at the peripheral portion of a photographing area with respect to the central portion thereof due to relation of an aperture efficiency of the illumination optical system and the photograph optical system (i.e., the aperture efficiency of the illumination optical system and the photographing optical system exerts influence). That is, in case a retina is photographed by using the conventional camera, an image of the retina photographed shows an excessive brightness at the central portion of the retinal image and a comparatively excessive darkness at the peripheral portion thereof. As seen, the conventional retinal cameras presented a problem that it is difficult to obtain a retinal photograph having a uniform brightness all over the photographing area of the retina of the eye to be tested.
Particularly, in recent years, since there is such a tendency as that the photographing angle of view of a retinal camera is widened, the afore-mentioned problem is closed up. That is, it is an important technical problem for a retinal camera of recent years to eliminate irregularity of brightness of a retinal image. To this end, an attempt is being made to correct an illumination light quantity on the retina of an eye to be tested by vignetting, wherein the diameter, position, etc. of a ring-shaped slit aperture provided to an illumination light optical system are altered.
However, the ring-shaped slit aperture provided to the illumination optical system is restricted based on removal of harmful reflection light and possible pupil diameter. Because of the foregoing, there is a limitation in correcting the image light quantity ratio per unit area of the central portion of the retinal image to be photographed and the peripheral portion thereof. Particularly, it becomes more difficult to effect such a correction as mentioned due to the tendency of wider photographing angle of view and photographable smaller pupil diameter.